1. Field of the Invention
The present invention relates to a method of manufacturing an optical fiber, and more particularly to an optical fiber in which sections where the chromatic dispersion at a given wavelength is positive and sections where the chromatic dispersion is negative at a given wavelength are longitudinally alternately provided.
2. Related Background Arts
A wavelength division multiplexing (WDM) transmission system can effect high speed, high capacity optical communications by transmitting multiple wavelengths of optical signals in a single transmission line. Because the transmission loss of a silica optical fiber is small at the wavelength band of 1.55 xcexcm, and optical amplifiers for amplifying optical signals at the wavelength band of 1.55 xcexcm are available, optical signals of the 1.55 xcexcm wavelength band are used in WDM transmission systems.
When there is chromatic dispersion at the optical signal band in an optical fiber transmission line used for a WDM transmission system, the pulse waveform of the optical signal is deformed and transmission quality deteriorates. Therefore, from this point of view, it is desirable that the chromatic dispersion value in the optical signal wavelength band is small. On the other hand, when the chromatic dispersion value in the optical signal wavelength band is substantially zero, the nonlinear optical effect of four-wave mixing tends to occur; thereby crosstalk and noise arise, and transmission quality deteriorates. In order to suppress the occurrence of four-wave mixing, the optical signal power might be reduced but such an optical transmission system would require the repeater spans to be shorter such that more optical amplifiers would need to be provided and hence it would be expensive.
In order to solve these drawbacks, it is proposed in U.S. Pat. Nos. 5,894,537 and 5,887,105 that the total dispersion of an optical fiber waveguide be managed to a target value by varying the chromatic dispersion from positive to negative and negative to positive at the wavelength of 1.55 xcexcm, alternately along the length of the waveguide.
If such dispersion-managed optical fiber is used for a transmission line, its average wavelength dispersion as a whole would be approximately zero and the deterioration of its transmission quality due to the wavelength dispersion can be restrained. Also, the chromatic dispersion of the transmission line is supposed to be made substantially not to be zero at most of the areas such that the deterioration of the transmission quality due to the four-wave mixing is restrained.
In the above American patent, the manufacturing method of such dispersion-managed optical fiber is also disclosed. According to this method, while an optical fiber preform having a longitudinally uniform refractive index profile is drawn, the diameter of the optical fiber is longitudinally altered so that the wavelength dispersion is adjusted to produce a dispersion-managed optical fiber.
However, because the chromatic dispersion of the optical fiber greatly depends on the core diameter, occasionally it cannot produce the longitudinal distribution of desired values. When the refractive index profile of the optical fiber preform is measured with a preform analyzer to determine the diameter of the optical fiber according to the measurement result, if its measurement precision is not sufficient, the chromatic dispersion of the optical fiber obtained by such drawing often fails to meet the desired value. Therefore, the transmission characteristic of the optical fiber often degrades. This is conspicuous especially in the case of the optical fiber whose effective core area is expanded or whose dispersion slope is reduced.
The purpose of the present invention, which is accomplished by solving the above-mentioned problem, is to provide the method of precisely manufacturing an optical fiber which has a desired chromatic dispersion.
The manufacturing method of the optical fiber according to the present invention comprises: (1) preparation of an optical fiber preform which has a longitudinally uniform refractive index, (2) measuring the chromatic dispersion of a predetermined length of the optical fiber obtained at the beginning of the drawing, (3) determining, according to the measured chromatic dispersion, the diameter of the optical fiber for each of the sections where the chromatic dispersion is to be positive and where the chromatic dispersion is to be negative, and (4) drawing the remainder of the optical fiber preform according to such determined diameter so as to produce an optical fiber having the alternating sections where the chromatic dispersion of a given wavelength is positive and where the chromatic dispersion of a given wavelength is negative. According to the manufacturing method, an optical fiber which has a desired chromatic dispersion can be precisely manufactured, even if the refractive index profile of the optical fiber preform cannot be precisely measured by the preform analyzer, or the like. Furthermore, the optical fiber thus obtained has excellent transmission characteristics because the waveform distortion due to the chromatic dispersion and the nonlinear optical effect can be restrained.
Another feature of the present invention is that the manufacturing method of the optical fiber enables to maintain a constant drawing tension by controlling the drawing speed or the temperature of the fused portion of the optical fiber preform for the remainder of the optical fiber preform. In this case, an optical fiber that has a desired chromatic dispersion characteristic can be precisely manufactured because the refractive index change caused by the residual stress does not vary longitudinally, and hence the change of the chromatic dispersion is caused only by the change of the diameter, i.e., the core diameter, of the optical fiber.
The above and further objects and novel features of the invention will be more fully clarified from the following detailed description when the same is read in connection with the accompanying drawing. It is to be expressly understood, however, that the drawing is for the purpose of illustration only and is not intended as a definition of the limits of the invention.